The 3rd Generation Partnership Project (3GPP), the standards group for asynchronous cellular mobile communication, is studying the support for wireless communication between terminals or devices, namely, Device-to-Device (D2D) communication using the Long Term Evolution (LTE) system standards, as well as conventional wireless communication between a base station and a terminal. In particular, one important function which 3GPP requires of D2D communication is a public safety service support function. That is, even in the case of emergency (such as in a natural disaster) in which a normal network service cannot be provided, the D2D communication has to be able to support LTE based wireless communication within or between police, firemen, and government agents.
In general, for smooth communication service support in a wireless communication system, it is necessary to acquire a timing reference to be used for signal transmission or reception of terminals in a system, namely, to acquire time synchronization. Also, in an LTE system, when a cellular network service is operating normally, terminals inside the service region of a base station can receive a Primary Synchronization Signal (PSS)/Secondary Synchronization Signal (SSS) transmitted through a downlink from the base station to acquire synchronization. However, in situations in which the cellular network service cannot be normally provided due to an emergency as described above, it is impossible to acquire synchronization from the base station.
Accordingly, there is a need for a synchronization acquisition means for supporting communication between devices even in situations in which cellular network service cannot be normally provided due to emergencies.